This proposal involves the analysis of pathways of morphogenesis and development by molecular genetic means. Two biological systems are to be used, the Salmonella Phage P22 and the yeast Saccharomyces cerevisiae. The plan is to develop and to test methods of 1. Classifying genes identified by conditional-lethal (cold sensitive (cs) and temperature-sensitive (ts)) mutations into the same, as opposed to parallel, pathways. 2. Determining the order of gene function by temperature-shift experiments in cs-ts double-mutant recombinants. 3. Finding methods for identifying which gene products interact at the molecular level. 4. Isolating specifically mutants in a pathway of interest through a chain of second-site reversions. The phage system will offer the advantage that the gene product interactions can be studied independently of the genetic inferences by physical and biochemical methods. The specific problems (morphogenesis of the P22 phage head and repressor-antirepressor interaction) will be pursued for their intrinsic interest as well as for their potential usefulness as tests of our genetic methods. The yeast system (genetic control of the yeast cell division cycle) will be the major new application of our methods. We plan to classify the 32 known cdc (cell-division-cycle) genes according to pathways and order of gene function and to isolate second-site revertants (gene-specific suppressors) as a clue to gene product interactions.